In cement clinker production, the raw material is first preheated, then precalcined and finally burnt in an oven. The actual combustion process takes place in the sintering zone of the oven, in which the material is heated to temperatures of from 1400° C. to 1500° C.
DE69806182 relates to a method for producing cement clinker by means of sulphur-rich fuel, the oxygen concentration being increased at the inlet of the combustion oven to from 4.5% to 5.5%, whereby the temperature at which the calcium sulphate breaks down is increased to a temperature greater than the sintering temperature so that CaSO4 becomes a component of the end product instead of breaking down into gases and leaving deposits in the combustion oven, the preheating oven and the preheating cyclones.
Specific components, particularly alkali metals in combination with chlorine and sulphur, become volatile in the sintering zone of the oven and are discharged with the oven exhaust gas (volatility). The oven exhaust gas is used to thermally process the material in the calcining zone and the preheating zone. During that heat exchange, the volatile components condense on the raw material (adsorption) and are again introduced into the sintering zone. There, they partially become volatile again so that a circuit of those components is thereby formed. Consequently, everything which is not discharged out of the system with the cement clinker or the exhaust gas remains in that circuit, very high concentrations of the components carried in the circuit being able to be reached. All the circuits may result in undesirable formation of deposits in the calcining zone and the preheating zone and disruptions to operation. The cause is that specific mixtures of the circuit components result in eutectic compositions which melt at comparatively low temperatures. When the raw material melts and becomes crystallised on the plant walls, deposits are formed.
A particular problem in this context is the sulphur which is introduced into the circuit via the raw material and the fuel. The maximum SO3 concentration allowed in the combustion material is therefore currently 5%. At higher concentrations, there is a risk of blockage and the plant can no longer be operated. If chlorine is further present, the tolerable quantity of SO3 is further reduced. In the illustration according to FIG. 1, the interaction of SO3 and Cl is illustrated on deposits in the inlet of the cylindrical rotary kiln. No deposits can be established in the region A whereas normal cleaning is necessary in region B and intensive cleaning is necessary in region C. There is a risk of blockage in the region D and at an SO3 concentration of more than 5%.
Therefore, attempts have previously been made to prevent circuits or volatility. In order to reduce the circuit concentration, it is known to provide a bypass which removes a portion of the oven exhaust gases. As a result, circuit components are removed from the circuit and the charges of the circuits are reduced. The circuit concentrations are thereby reduced and formations of deposits diminished, the availability of the plant thereby being improved.
However, there are fuels which have a relatively high sulphur concentration and could not therefore previously be used in cement production. Although EP-A2-1428804 sets out a cement clinker which is intended to be produced with fuel which has a sulphur concentration of more than 5%, the patent specification does not set out in greater detail how that cement clinker can be produced without resulting in formations of deposits in the preheater or calciner or increased SO2 emissions.